The present invention relates generally to digitizing tablet systems and more particularly to digitizing tablet systems which include a conductive grid structure and a stylus structure which is inductively coupled to the conductive grid structure.
Digitizing tablet systems are well known in the art and are used in a variety of applications. These systems generally include a tablet, a stylus and some instrumentality for producing some form of interaction between the stylus and the tablet from which is derived data signals representing the position of the stylus on the tablet.
In a number of these systems the tablet includes a housing having a two dimensional work support surface and a grid of conductive elements and the stylus contains an electric coil. The coil in the stylus is inductively coupled to the grid in the tablet by energizing either the coil or the grid with an AC voltage signal. The voltage signal induced in the other component is then measured. The measured signals are then used to determine the position of the stylus relative to the grid.
The grid in inductive coupling type digitizing tablet systems very often comprises a first set of parallel equally spaced straight wires and a second set of parallel equally spaced straight wires which are at right angles to the first set of wires. One end of each wire in each set is connected to a common conductor having one end which serves as one terminal for the set. The other end of each wire in the set is connected to a multiplexor or other similar device having an end which is connected to a conductor which serves as the other terminal for the set. The multiplexor allows the individual wires in the set to be enabled sequentially. The two terminals for each set are coupled to the electronics portion of the system where signal processing is performed. By energizing the coil in the stylus, voltages will be induced in the individual wires or, alternatively, by energizing the individual wires, voltages will be induced in the coil in the stylus. The induced voltage signals are processed to produce data signals representing the position of the stylus relative to the grid.
In the past, it has been considered that the size of the induced voltage signal is directly related to the proximity of the coil in the stylus to the particular wire in the grid from which the induced voltage signal is derived. The problem with this line of reasoning is that it is not entirely correct. More specifically, the induced voltage generated in or from a wire is not related exclusively to the proximity of the coil in the stylus to the particular wire but rather to the overall loop defined mainly by the particular wire in the grid wire but also including the common conductor, the multiplexor and the connections from the common conductor and the multiplexor to the associated electronics. Since the individual wires are straight, equally spaced from one another, and connected at opposite ends to common elements, the size or area of the overall loop related to each individual wire will be different even though the distance between wires is the same. Because the loops are differently sized, the induced voltages related to each wire will be different. For example, assume a voltage signal of 1 volt is applied to the coil of the stylus and that the stylus is positioned 0.1 inches from a wire in the grid which forms part of an overall loop having an area of 12 square inches and that the induced voltage detected by that wire is 0.1 volts. If the stylus is positioned 0.1 inches from another wire in the grid which forms an overall loop having an area of 80 square inches, the induced voltage detected by that wire will not be 0.1 volts but will be somewhat smaller, such as 0.4 volts.
In addition, as the size of the overall loop increases, the amount of stray fringe fields picked up by the loop also increases.
As can be appreciated, both of the above phenomena will produce nonlinearities or errors in the relationship between the induced voltage signals and the computed stylus position which in many applications are undesirable and/or unacceptable.
In U.S. Pat. No. 4,243,843 to R. T. Rocheleau there is disclosed an apparatus for determining the distance of a point on a surface from an axis on the surface comprising a cursor having a magnetisable element adapted to be energized by an alternating voltage, a grid of substantially parallel spaced conductors, the cursor being movable thereover and inductively coupled thereto, means for sequentially enabling only one of said conductors to conduct a current induced by the alternating voltage at any one time, means for comparing percentages of the amplitudes of currents induced in successively enabled conductors, means for sensing a change in the phase of the induced current, and means responsive to the comparing means and the sensing means for counting the number of conductors which have been enabled at the time a phase change is sensed.
In U.S. Pat. No. 3,873,770 to J. T. Ioannou there is disclosed a system for digitizing graphic data from a worksheet by tracing out or pointing to curves and points on the worksheet with a stylus. A tablet having a surface to receive the worksheet includes a conductor grid defining two perpendicular axes of measurement. The conductors are sequentially excited and a coil in the stylus picks up an impulse having an envelope which shows positive and negative peaks spaced by a distance 2h, where h is coil height above the grid plane measured along the stylus axis. The conductor grid planes are physically displaced from the tablet surface by some small but finite distance thus giving rise to an apparent position error if the stylus is tilted during use; i.e., the indicated position will be the projected intersection of the stylus axis with the grid plane, not the surface. Means are provided for detecting stylus tilt and variations in h and to compensate for position errors.
In U.S. Pat. No. 3,647,963 to K. N. Bailey there is described a device for determining position coordinates of points on a surface which includes a conducting grid structure having at least two grid elements to be placed over or under a surface and a cursor structure having a circular conducting loop element to be moved across the surface of the grid structure. An alternating electric signal is supplied to either the cursor conducting loop or to each of the conducting grid elements. This signal induces a signal in each element of the unexcited conducting structure. Position coordinates are determined by apparatus which measures the induced signal or signals and records the signal change produced when the cursor is moved across the grid surface. Several embodiments of measuring devices which determine the distance between arbitrary points on a surface such as a map, graph or photograph are illustrated. Automatic plotting embodiments are also shown and described in which the plotting motion is determined by comparing signals representing the measured loop position on the grid with a preselected set of command signals.
In U.S. Pat. No. 3,705,956 to M. L. Dertouzos there is described a graphic data tablet comprising a tablet having first and second opposite faces; at least first and second conductor means mounted to said first face of said tablet, said first conductor means disposed in a first path corresponding to a first bit in the Gray Code, said first path occupying a first area of said first face, said second conductor means disposed in a second path corresponding to a second bit in the Gray Code, said second path occupying a second area of said first face, said first and second areas being separate and distinct, said first and second conductor means being coplanar; means for sequentially passing a signal through said first and second conductor means; and sensing means electromagnetically communicating with said first and second conductor means for sensing said signal sequentially passed through said first and second means.
Other known patents of interest include U.S. Pat. Nos. 3,974,332 to Abe et al; 4,368,351 to S. E. Zimmer; 3,975,592 to P. C. Carvey; 3,999,012 to H. Dym; 3,700,809 to D. J. Nadon; 3,732,369 to W. L. Cotter; 3,735,044 to R. M. Centner; 3,399,401 to T. O. Ellis et al; 4,213,005 to E. A. Cameron; and 4,289,927 to J. L. Rodgers.
It is an object of this invention to provide a new and improved digitizing tablet system.
It is another object of this invention to provide a new and improved inductive coupling type digitizing tablet system.
It is still another object of this invention to provide a new and improved grid structure for an inductive coupling type digitizing tablet system.
It is yet still another object of this invention to provide a grid structure for an inductive coupling type digitizing tablet system which does not produce nonlinear relationship between induced voltage signals and the computed stylus position.
It is a further object of this invention to provide a grid structure for an inductive coupling type digitizing tablet system in which interference from stray magnetic fields is minimized.
It is another object of this invention to provide a grid structure for an inductively coupled digitizing tablet system in which the overall loops containing the conductive elements are substantially equal in size and shape.
It is still another object of this invention to provide a technique for interpolating induced voltage signals derived from a grid of uniformly sized and spaced U shaped loops of conductive material to determine position.